A reference interval provides information about a range of measurements observed in the reference population to assist health care providers in interpretation of individual clinical laboratory test results.
Existing regulations require laboratories to provide reference intervals on test result reports and review/revise those intervals on regular basis. Many laboratories adopt reference intervals from other sources, such as other laboratories, manufacturers of testing reagents, or previously published studies. In 2008, the Clinical and Laboratory Standards Institute-approved guideline recognized the reality that, in practice, very few laboratories perform their own reference interval studies, instead referring to studies done many decades ago, when both the methods and the population were very different. (Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory; Approved Guideline. Third Edition. CLSI document C28-A3. Wayne, Pa.: Clinical and Laboratory Standards Institute; 2008). Thus, it is apparent that many reference intervals which have been reported for decades may not currently be accurate for a given laboratory due to differences in modern testing methodology and/or the population serviced.
There are a number of additional problems contributing to resistance or reluctance to change current practice. Conducting independent de novo studies for reference interval determinations using the conventional direct donor sampling method is expensive and has limitations and complications. The studies typically recruit healthy subjects, whereby criteria must be defined for determining which subjects are “healthy.”
Recruiting and obtaining informed consent from candidate subjects and excluding subjects with subclinical diseases can be difficult and expensive. Moreover, the healthy reference populations likely include subjects with subclinical disease. Even successful studies of this type have relatively low sample sizes (e.g. about 100-150 individuals), such that statistical power is lacking. It is statistically more robust to analyze thousands of measurements that include a number of unhealthy subjects than 120 subjects assumed to be healthy. Large sample size is essential for accuracy in determination of reference intervals.
An indirect method of reference interval estimation that used test results already stored in the laboratory database was described by Hoffmann in 1963 (Hoffmann, R G. Statistics in the Practice of Medicine. JAMA, 185: 864-873, Sep. 14, 1963). Hoffmann described a method using manual plotting of test data on graph paper and visual assessment of the graph for reference interval estimation. It was limited by subjectivity of visualization and manual data manipulations. Manual and semi-manual data manipulations using Hoffmann's method were also used in later publications (Soldin et al. Pediatric Reference Intervals, AACC Press, 6th edition).
To better serve the healthcare industry, the clinical laboratory industry is in need of robust and reliable methodology for determination and verification of reference intervals for clinical laboratory test results.